The invention relates to the use of a specific catalyst in aromatization reactions of hydrocarbons having between 2 and 4 carbon atoms per molecule.
The catalyst incorporates an MFI structure zeolite, which contains silicon and aluminium, as well as at least one noble metal from the platinum family, to which are added at least one additional metal chosen from within the group consisting of tin, germanium, indium and lead. An amorphous matrix can be added to the catalyst with a view to the shaping thereof.
The aromatization reaction of propane and butane in the presence of a catalyst containing the ZSM5 (or MFI) zeolite and gallium was discovered and patented by British Petroleum (U.S. Pat. No. 4,175,057 and U.S. Pat. No. 4,180,689). Since this data other companies and organizations have filed patent applications concerning modifications to the solid (U.S. Pat. No. 4,795,844, European patent EP-B-0252705) and/or to changes in the charge; C.sub.2 -C.sub.12 fraction (European patent EP-B-0252705), ethane and ethylene (EP-B-0050021 and U.S. Pat. No. 4,350,835). The way in which the gallium is introduced has also been covered by patents (European patents EP-B-0120018 and 0184927).
The addition of another metal to the Ga-MFI system has also been envisaged with a view to improving the selectivity for aromatics aromatics selectivity and for reducing the coke quantity on the catalyst. Thus, the impregnation of a Ga/MFI catalyst by rhenium associated with platinum or palladium makes it possible to significantly improve the aromatics production selectivity (U.S. Pat. No. 4,766,265). Other patents claim the addition of platinum and palladium to the Ga-MFI catalyst (U.S. Pat. No. 4,766,265, European patents EP-B-0215579, 0216491, 0224162, 0228267 and Japanese patents 61268634, 62081329).
The addition of platinum to the MFI zeolite makes it possible to improve the conversion of propane (T. INUI, F. OKAZUNI, Y. MAKINO, Chem. Express 1 (1), 53-56, 1985). However, the methane and ethane production selectivity is significantly increased. The addition of rhenium to the platinum further accentuates the production of methane and ethane by hydrogenolysis (S. ENGELS et coll., Catalysis Today, 3, 437-443, 1988). The addition of copper (P MERIAUDEAU et coll., Zeolites: Facts, Figures, Future, 1423-29, 1989) and chromium (E. S. SHPIRO et coll. International Symposium on Zeolites as catalysts, Sorbents and Detergent builders, Wurzburg (GFR), p 73, 1988) reduces the production of methane, but the aromatics selectivity remains inferior to Ga/MFI systems. Moreover, the addition of sulphur to a Pt.MFI catalyst makes it possible to significantly improve its aromatics production selectivity on the basis of paraffins containing 6 to 12 carbon atoms (U.S. Pat. No. 4,835,336).
In French patent application 90/06 557, there is claimed a catalyst based on the associated between an MFI zeolite and a generally amorphous support or matrix, on which is deposited a noble metal from the platinum family and at least one additional metal chosen from within the group consisting of tin, germanium, indium and lead, said support also containing at least one alkali metal or alkaline earth metal chosen from within the group consisting of lithium, sodium, potassium, rubidium, cesium, barium, calcium, beryllium, magnesium and strontium. An improvement is described in the performance characteristics compared with what was known from the prior art in connection with the aromatization of light hydrocarbons.